"The Spiral of Merlyn – these are the narrow stairs carved into the stone beneath Hogwarts that lead down into the harrowing labyrinth of the dungeons. This was one of the hidden entrances to the Chamber of Secrets." ~SS
Hogwarts appears to the wizarding world as an intact castle of architectural interest. To the Muggle world it is a ruin of little interest. So dull that the hapless Muggle that stumbles across the ruins he/she will not even photograph it. They are unremarkable ruins that the Muggle eventually forgets. This is the benefits of a Fidelius Charm masterfully interwoven with an Architectural Glamour and an Unplottable Spell.
Hogwarts, since its first stone was laid down by the Founders, has suffered Natural disaster, various accidental fires and blasts from learning students, deliberate fires from angry students and terminated staff, and damage from war. Not just from the Final Battle you have read about in our histories but it suffered damage from a few Goblin Wars and from the monumental battle between Gellert Grindewald and Albus Dumbledore. Although Hogwarts has suffered, never has any witch or wizard since the Founders ever re-built it. It is sentient enough that She will rebuild herself. However, rooms, attics, storerooms, dungeons, tunnels, and more were simply... well, not entirely simply... were added on via stone manipulation, bequests of libraries to full rooms, and a variety of magical effluence.
The Spiral of Merlyn is old enough to have qualified for the first addition but in reality the Chamber of Secrets was first addition; the entrances came later.
The Chamber of Secrets was devised by Salazar Slytherin but built with the help of Rowena Ravenclaw who knew how to manipulate stone to her will. The Chamber was designed to hold Salazar's grand library of tomes, journals, and scrolls he had collected the world over. This library would also include those manuscripts found in the Muggle world and written by Muggle-Men of amazing intellect. As to the basilisk that Harry Potter found? No knows who brought it there as it was of an age that pre-dated the foundation of Hogwarts. It is possible that Salazar Slytherin owned the magical beast however we do not know. Perhaps that is knowledge in the Journals of Salazar Slytherin; penned in his own, interpretation of Parseltongue. Parseltongue has no written word.
The main entrance is the one that existed within the second floor girls bathroom under the sinks. [This entrance was later collapsed by Albus Dumbledore.] This entrance did not move or change as Salazar Slytherin had fixed it in place with Parseltongue Magic. Only an heir, or someone inherited of Parseltongue, would be able to gain ingress.
The Spiral of Merlyn was created by Sir Terence du Sang. He was a Slytherin who became a knight in the army of Rowena Ravenclaw where he met, and fell in love with her daughter, Helena.
Now, many know the sordid story of how Sir Terence became the Bloody Baron after succumbing to a madness in which he murdered Helena Ravenclaw for spurning his love, and then he fell upon his own sword in grief. What is not known is that Sir Terence was an apprentice to Rowena Ravenclaw and learned her magical skill of 'In-Organic Manipulation'; a precursor to the more thorough discipline of Transfiguration.
Sir Terence, with Salazar Slytherin's blessing, created the Spiral of Merlyn. The stairs, an impressive number of 48, was hewn, or created from, the bedrock beneath the castle. The stairs did not lead directly to the Chamber of Secrets but right at their middle, the 24th step, there was a particular set of stones in the wall that, if the witch or wizard exploring knew the Parseltongue charm, could reveal a wall that led into the Chamber.
The Spiral of Merlyn was to serve the purpose of becoming the Gateway to rooms, chambers, tunnels, and more, all beneath the castle. Of course, with the death of Sir Terence the Spiral of Merlyn went nowhere after its landing at the end of the 48 steps.
By the time of the appearance of The Bloody Baron haunting the corridors and halls of Hogwarts the splendid Attics were beginning to take upon their curious form beginning with the appearance of The Copernican Observatory. Wizards and witches near and far were bequeathing their libraries and sometimes entire rooms, labs, and 'quarters of note' to Hogwarts. The additions were all becoming new Attics whereas the Darker wizards and witches were sending their rooms, journals, or objects for the Dungeons Beneath the Dungeons as accessed via The Spiral of Merlyn.
With these arriving bequests it was Doromanicus Black, the first instructor of Defense Against the Dark Arts who completed work upon The Spiral of Merlyn. He then hid the entrance via a Moving Wall that had to be Summoned before it could be "unlocked". Doromanicus, who was rather wickedly skilled in the application of Magical Riddles, used a Dark Charm to constantly change the riddle to open the door. It is not a simple tapping of stone bricks in the right order.
If one can Summon and open the door to The Spiral of Merlyn the Dungeons Beneath the Dungeons will bring much enrichment to the intrepid explorer.
END
a/n: Thank you to slytherinsal for the correction on using Sir Terence vs. Sir du Sang